A new ultrasonic angle-beam technique for sensitive evaluation of closed cracks

This paper is on the ultrasonic nondestructive evaluation of smaller cracks in the size range of 1-7 mm. Sensitive evaluation of the crack that was or iginally present or that developed during operation is imperative for asses sing the integrity of structural components. In order to develop a method f or the enhancement of sensitivity of ultrasonic evaluation of closed cracks , the principle of a new angle-beam approach is proposed, where the use of an oblique longitudinal wave with a small angle of incidence upon the speci men surface is emphasized. The crack treated exists nearly vertically on th e back surface of the specimen. In contrast with the normally incident wave beam, the effect of angular incidence on the ultrasonic response due to a crack is investigated in an attempt to predict the optimum angle of evaluat ion. The sensitive feature of the present angle-beam technique is discussed in light of comparison made with the usual methods of normal incidence and large-angle beam technique. Ultrasonic measurements performed on the close d fatigue-cracked specimens show that the method offers bright prospects fo r the sensitive evaluation of small, closed cracks. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.